This application claims the priority of German patent document 198 27 878.0, filed Jun. 23, 1998 and PCT International Application No. PCT/EP99/03492, filed May 21, 1999, the disclosures of which is expressly incorporated by reference herein.
The present invention relates to a circuit for measuring the voltage output from a composite fuel cell system.
An H2/O2 composite fuel cell system comprises a series circuit formed by a plurality of individual fuel cellsxe2x80x94typically on the order of 100. Each of these individual fuel cells supplies the rated current IN of the composite fuel cell system; and the sum of the voltages of the individual fuel cells produces the rated voltage UN of the composite fuel cell system, which can be tapped off as the total voltage at the terminals of the first and last individual fuel cells of the composite fuel cell system. Intermediate voltages can be obtained by mounting additional terminals mounted between the individual fuel cells.
Electrical circuits requiring an electrical energy supply are used for monitoring individual fuel cells, and for other control tasks. Such energy can be supplied by means of an external battery in the known prior art. If on the other hand, energy is intended to be supplied to the circuit from the composite fuel cell system itself, then this depends on the function of the circuit. That is, if it processes signals which do not originate directly from the fuel cell, then this energy can be obtained from the total output of the composite fuel cell system. By contrast, if signals of the composite fuel cell system are used directly, then it is not possible to supply current to the circuits without additional circuitry, owing to the necessary potential isolations. The signals of the composite fuel cell system may be, for example, the voltages of the individual fuel cells; in this manner it is possible to implement suitable measures in the event of a defect in individual cells, which is manifested in a significant change in their voltage.
In the case of a voltage supply from the composite fuel cell system, the measurement of the voltages of the individual fuel cells requires either a measuring device with ground-free input or a high permissible input voltage range.
The supply voltages of electrical circuits are typically of an order of magnitude of 5 to 15 V, with both negative and positive supply voltages being required for operational amplifiers, for example. Depending on the type of operational amplifier, it is also possible to work only with positive supply voltages (e.g. LM324).
If the composite fuel cell system is intended to be used for supplying voltage to the circuit, then problems arise. The input signal (in other words the measurement signal), which is at the potential xcfx86a relative to a reference signal xcfx86c, depends on the individual fuel cell to be measured. Particularly when processing cell signals of the upper cells, the input must then be especially voltage-stable in order e.g. to process a difference signal. The interaction of measurement signal, electrical protective circuit and its current supply turns out to be difficult, which is why the circuitry required for this purpose is complicated.
By way of example, if a subtracting amplifier is used, it is necessary to choose an operational amplifier which allows input voltages up to the total voltage of the composite fuel cell system (that is, approximately 200 V), and also additional overvoltage protection, even if the voltage to be measured is only in the vicinity of the supply voltage. However, the maximum actual input voltage is typically in the vicinity of the supply voltage of the amplifier (that is, approximately 15 V). Furthermore, it is another disadvantage that the amplifier operates at its drive-level limit in the case of this voltage supply from the composite fuel cell system in the event of driving with signals from the fuel cells in the edge region of the composite fuel cell system.
It is apparent from the above, therefore, that, to measure the potentials at individual terminals of the composite fuel cell system, it is necessary to use a measuring device which is either provided with a ground-free input or in which the permissible input voltage is at least the total voltage of the composite fuel cell system.
Furthermore, British patent document GB-A-2051382 discloses measuring the voltage of a battery by using a diode (or a zener diode) to obtain a reference voltage from the voltage to be measured; the reference voltage remains at least virtually constant, even if there is a drop in the battery voltage to be measured. The battery reference voltage thus obtained is at a lower level than the voltage to be measured. In parallel with this circuit in which the reference voltage is obtained, the voltage to be measured is shifted so that when a specific voltage level of the battery is undershot, the sign of the difference between the shifted voltage and the reference voltage changes. The fact that a specific voltage level of the battery has been undershot can be derived from the change in sign of this difference.
U.S. Pat. No. 3,997,888 discloses a circuit which is supplied with a stabilized voltage from a battery by a series circuit of a zener diode with a resistor connected in parallel therewith.
One object of the present invention is to provide simplified circuit arrangement of the type described above.
This and other objects and advantages are achieved by the measurement circuit according to the invention, in which a potential shifting element is interposed between the particular point of the composite fuel cell system and the input of the arrangement. The shifting element is connected in series with a current sink or current source, and potential shifting by the potential shifting element is dependent on the potential of the particular point relative to a reference point of the composite fuel cell system, given a proper state of the individual cells of the composite fuel cell system.
This makes it possible also to evaluate voltage signals from individual fuel cells which are situated at the edge of the composite fuel cell system.
The potential can then advantageously be adapted to an input voltage range.
In one embodiment of the invention, the voltage difference between two points within the composite fuel cell system is detected by tapping the potentials of the two points and feeding them to an arrangement for evaluating the voltage difference. Respective potential shifting elements are interposed between both points of the composite fuel cell system and the inputs of the arrangement, and each such element is also connected in series with a current sink or current source.
As a result, the voltage of an individual fuel cell can be detected as a voltage difference in a simple manner; for example, by shifting the two potentials by the same amount. This makes it possible to evaluate voltages of individual fuel cells which are situated at the extremities of the composite fuel cell system. The shifting of the potentials of the two points by the same amount may be performed, for example, by the respective potential shifting elements.
As a result, the voltage difference between the xe2x80x9cshiftedxe2x80x9d potentials and the xe2x80x9cunshiftedxe2x80x9d potentials is identical, so that no further outlay need be expended in order to take account of the potential shift when evaluating the voltage.
In another embodiment of the invention, the voltage measuring circuit comprises one or more integrated circuits, which achieves a cost advantage, particularly in large numbers.
In still another embodiment, at least one voltage stabilization element is connected in parallel with the voltage measuring circuit, and at least one current source or current sink is connected in series with this parallel circuit.
One advantage of this arrangement is that it is possible to supply voltage to an amplifier with positive and negative supply voltage, while at the same time it is also possible to generate a virtual ground that it is adjustable as desired. Current is supplied from part of the total voltage of the composite fuel cell system. This means, in particular, that no additional auxiliary source is required.